This invention relates to a circuit unit, mounted, for example, on a door panel of a vehicle or the like, and to a method of producing the same, more particularly relates to a circuit unit and a method of producing the same, in which a flexible printed circuit is used as a wiring circuit member on which electronic parts are mounted by soldering, and the flexible printed circuit having the electronic parts mounted thereon is received and held in a casing divided into two (upper and lower) sections, that is, a lower casing and an upper casing.
As power-window circuit units for opening and closing window glass panes of a vehicle, there have been developed various circuit units of the type which are mounted on a door panel, and include a wiring circuit member on which electronic parts are mounted by soldering and a casing which is divided into two (upper and lower) sections, that is, a lower casing and an upper casing, and has the wiring circuit member received and held therein and switches for opening and closing circuit contacts within the casing are provided on an upper surface of the upper casing.
In such a circuit unit, for example, the surface of the upper casing on which the switches are provided is, in many cases, formed into a curved structure in order to enhance the operability of the switches and also to match its design with the interior design. In order to enhance the ability of reception into the casing of a curved structure, the wiring circuit member to be received in the circuit unit is required to have flexibility so that an undue force will not act on this wiring circuit member arranged in conformity with the curved surface with in the casing. In addition to the electric circuits for the power windows, various electric circuits, for example, for adjusting power seats and for controlling courtesy lamps are incorporated in a door panel or the like of the vehicle. Therefore, in order to achieve a high packaging density in a limited space within the door, the wiring circuit member used in the circuit unit is also required to have a space-saving design.
In order to meet these requirements, there have recently been extensively used flexible printed circuits (hereinafter referred to as xe2x80x9cFPCxe2x80x9d) as a wiring circuit member on which electronic parts are mounted by soldering, and this flexible printed circuit has a wiring circuit printed on a film-like or a sheet-like thin flexible insulative substrate.
FIGS. 6A to 6E show the process of producing a related circuit unit using an FPC as a wiring circuit member on which electronic parts are mounted.
The production process shown in here is a process to the step in which the FPC, having the electronic parts mounted thereon, is received and held in a lower casing of the circuit unit.
More specifically, first, the FPC 1 on which the electronic parts are to be mounted is attached in position to an exclusive transport support plate 3, as shown in FIG. 6A. The flat surface 3a of the transport support plate 3 is brought into close contact with the lower surface of the FPC so that the FPC which the electric parts are to be mounted on the upper surface thereof is supported by the transport support plate 3, thereby preventing flexible displacement of the FPC 1 which would invite an erroneous processing during the electronic parts-mounting operation and the solder paste-coating operation. A mold release material is provided on that region of the transport support plate 3, indicate by hatching in the drawings, so that the FPC 1 can be easily separated or peeled therefrom when removing it from this plate at a later step.
Then, the solder paste 6 is coated onto electric part-mounting portions of the upper surface of the FPC 1 by a squeegee 5, as shown in FIG. 6B. The solder paste 6 is coated only to electric part-soldering portions through paste introducing holes 7a formed through a masking member 7 superposed on the upper surface of the FPC 1.
The predetermined electronic parts 9 are attached to the upper surface of the FPC 1 coated with the solder paste 6, and then a holder plate 11 is superposed on the FPC 1 as shown in FIG. 6C, and in this condition the FPC 1 is charged into a solder reflow furnace so as to solder the electronic parts 9.
The holder plate 11 has a window 11a through which that region of the FPC 1 to which the electronic parts are attached (that is, that region where the solder paste 6 is coated) is exposed, and this holder plate holds the non-soldering region in close contact with the transport support plate 3.
The FPC 1, which has been charged into the reflow furnace and the soldering of the electronic parts 9 has been finished, is separated from the transport support plate 3 as shown in FIG. 6D. Then, this FPC is fixed in position to an upper surface of a bottom wall 13a of the lower casing 13 of the circuit unit, as shown in FIG. 6E.
In the above related production method, during the FPC 1 to which the parts have been soldered is separated from the transport support plate 3 as shown in FIG. 6D, a stress is applied to the solder joint portions because of curvature of the FPC 1 or the like, and this leads to a possibility that the electrical and mechanical performance of the solder joint portions is lowered.
Another problem is that the separating operation must be carried out carefully in order to reduce the stress, applied to the solder joint portions due to the curvature of the FPC 1 or the like at the time of separating the FPC 1 from the transport support plate 3, to a minimum, so that the productivity is lowered.
Further, the exclusive transport support plate 3 needs to be designed and produced in accordance with the dimensions of the FPC 1 and so on. Therefore, there has been pointed out a problem that the design cost and production cost of the transport support plate 3 increases the production cost of the circuit unit.
Furthermore, in the above related production method, the FPC 1, on which the electronic parts 9 have been mounted by soldering, is fixed in position to the upper surface of the bottom wall 13a of the lower casing 13 as shown in FIG. 6E. However, that portion of the FPC, on which the electronic parts 9, are mounted, is considerably reduced in flexibility, and besides this portion can not be accidentally pressed in order to reduce the stress, applied to the solder joint portions, to a minimum. As a result, the FPC 1 is mounted on the lower casing 13 in such an unstable condition that part of the parts-mounted portions of the FPC 1 are lifted off the lower casing 13, and when switches and so on are mounted on the FPC 1 through a rubber contact or the like, there is a possibility that the reliability of the operation of the switches is lowered because of the lifting-off of the FPC 1.
This invention has been made under the above circumstances, and an object of the invention is to provide a circuit unit, as well as a method of producing the same, in which the production cost can be reduced by eliminating the use of an exclusive transfer support plate in a soldering operation and so on, and besides the productivity can be enhanced by eliminating the steps of attaching and detaching an FPC relative to such a transfer support plate, and further the application of stresses to solder joint portions by a handling operation during the production process is considerably reduced, and at the same time the close contact of the FPC with a lower casing is enhanced, thereby enhancing the electrical and mechanical performance of the solder joint portions, and furthermore when switches are mounted on the FPC, the reliability of the operation of the switches can be enhanced.
In order to solve the aforesaid object, the invention is characterized by having the following arrangement.
(1) A circuit unit comprising:
a flexible printed circuit including a wiring circuit formed on an insulative substrate thereof, an electronic part being mounted on an upper surface thereof;
a lower casing for holding the flexible printed circuit, including a flat bottom wall for covering a lower surface of the flexible printed circuit, wherein the lower casing is made of a heat-resistant material so that the lower casing can be sent to a solder reflow furnace in a state that the electric parts is mounted on the flexible printed circuit; and
a upper casing, for covering the flexible printed circuit, including an engagement member through which the upper casing is connected to the lower casing.
(2) The circuit unit according to (1), wherein a lower surface of the bottom wall to which the flexible printed circuit is not mounted includes a reinforcing rib for preventing warp of the lower casing.
(3) The circuit unit according to (2), wherein the reinforcing rib is integrally formed with the lower casing.
(4) The circuit unit according to (1), wherein the upper casing is provided with a switch key top for operating a switch formed on the flexible printed circuit.
(5) A method of manufacturing a circuit unit comprising a flexible printed circuit, a lower casing for holding the flexible printed circuit, and an upper casing, for covering the flexible printed circuit, connectable to the lower casing, the method comprising the steps of:
putting and fixing the flexible printed circuit on a predetermined flat position of the lower casing;
mounting an electric part on the flexible printed circuit;
sending the lower casing, mounting the flexible printed circuit with the electric part, to a solder reflow furnace so that the electric part is soldered to the flexible printed cable; and
connecting the upper casing to the lower casing.
(6) The method according to (5), wherein in the putting and fixing step, a positioning jig positions the flexible printed circuit on the predetermined flat position by passing projections of the positioning jig through the lower casing and the flexible flat circuit.